1. Field of the Invention
This invention relates to a control system for a vehicle automatic transmission, and more particularly to a control system for a vehicle automatic transmission which improves the gear shift control characteristics during hill climbing and descent.
2. Description of the Related Art
Most control systems for vehicle automatic transmissions use a gear shift scheduling map prepared in advance and the gear to be selected is determined by retrieval from this map using as address data an operating condition parameter indicating the engine load (e.g. the throttle opening) and an operating condition parameter indicating the vehicle speed. However, the gear shift map is based on the most commonly encountered driving conditions and is therefore not always able to provide the best gear shift characteristics under special conditions, as when driving up or down hill, for instance. A system that has been proposed for overcoming this problem is that taught by Japanese Patent Publication No. Sho 59-8698. This improved system uses separate gear shift scheduling maps (gear shift diagrams or programs) for different road profiles--one for level-road driving, another for up-hill driving, and so on. A calculation is made based on operating condition parameters such as the throttle opening and vehicle speed for predicting the value of an index of the running resistance of the vehicle on a level road. A concrete example of the index is the vehicle acceleration. The predicted acceleration is then compared with the actual acceleration calculated from the vehicle speed, and the gear to be selected is determined from the gear shift scheduling map based on the result of the comparison.
Control of this type requires the vehicle acceleration (more generally the index of running resistance) to be calculated from the engine load and other parameters with a high degree of accuracy. However, time lags in the air intake system, fuel supply system, drive train and the like produce a time lag between the occurrence of a change in engine load and the time when this change appears in the power output torque. The length of this time lag is particularly large when the engine load changes rapidly. Since it is therefore impossible to determine the index of running resistance with high accuracy during rapid changes in engine load, the gear shift control is apt to become inappropriate.
Such a difficulty in determining the index of running resistance with high accuracy also arises when a brake pedal is depressed, since engine output torque decreases in response to the braking force. Also the same will happen when gear shifting is in progress, i.e., when application and release of gear clutches takes place, or when any apparatus such as an air conditioner is equipped with which is to be driven by the vehicle engine, since engine output torque is partially consumed by the apparatus. The same also occurs when drive wheels slip which makes a vehicle speed different from a true value if the vehicle speed is detected from the rotational speed of the drive wheels. Or when range gear positions "D, D3, 2, . . . " are being switched therebetween, gear shift control characteristics will then be changed so that running resistance would be unstable.
It is therefore one object of this invention to provide a control system for a vehicle automatic transmission which overcomes the aforesaid drawbacks of the prior art system and is able to prevent erroneous control based on erroneous determination of the index of running resistance.
The prior art system also has shortcomings in that the need to predict the vehicle acceleration separately for each gear position makes them intrinsically complex and in that the method it employs for switching between different gear shift characteristics in response to the difference between the predicted and actual accelerations is apt to result in instable control in which the control value changes frequently, particularly during times when the vehicle operating conditions change rapidly.
It is therefore another object of the invention to provide a control system for a vehicle automatic transmission which eliminates these disadvantages of the prior art system.
Moreover, as mentioned above, braking will affect the determination of the index of running resistance. As regards the braking, due to a clearance between a brake disc and a brake pad or due to a play of a brake pedal, there is a time lag in the brake pedal depression and braking force resulted therefrom and thus, the braking force remains for a period after the brake pedal was released. The braking signal is frequently used in many controls including automatic transmission control and since it is expensive to prepare a sensor which can detect braking condition through hydraulic pressure of an brake oil, for example, such a sensor is usually used to detect braking condition as is turned on when a brake lamp is switched on or when the brake pedal is depressed. As a result, when the latter type of sensor is used, it could erroneously detect that braking condition ends at a time when the brake pedal is released.
It is therefore further object of the invention to provide a control system for a vehicle automatic transmission which eliminates the aforesaid shortcomings.
Further, as mentioned earlier, the index of running resistance could not be determined accurately when gear shifting is in progress. On the other hand, if a vehicle stops temporarily during hill descending and then starts once again, gear should preferably be shifted up to the top in a short period when vehicle continues rapid acceleration. Therefore, if the gear shift control is discontinues each time gear shifting is in progress, upshifting could be delayed, resulting another problem.
It is therefore still further object of the invention to provide a control system for a vehicle automatic transmission which solves the problem just mentioned above.
Furthermore, slippage in drive wheels could bring a problem in determining the index of running resistance accurately as referred to earlier. A traction control system is often introduced to control engine output in a situation such as the drive wheel's slippage. And if the traction control system is provided with and if the traction control system operates frequently, a road on which the vehicle travels could be quite different from that the aforesaid gear shift scheduling maps do expect.
It is therefore still further object of the invention to provide a control system for a vehicle automatic transmission which copes with a traveling condition in which the traction control system operates frequently.
Furthermore, when driving down hill, a gear shift scheduling map for hill descending is selected in the prior art system to facilitate hill descending by providing a better engine braking response. (The term "engine braking" or "engine brake" appeared in the specification means a condition in which a vehicle engine is driven by vehicle wheels so that the vehicle slows down its speed.) However, when running down a gentle or moderate slope, the necessity of engine braking is not so high and what is more, the engine speed becomes greater in low gear than that in high gear. This could make engine noisy, degrading a drive feeling.
It is therefore yet still further object of the invention to provide a control system for a vehicle automatic transmission which eliminates the disadvantage of the prior art system.